A liquid crystal display (referred to herein as LCD) generally consists of several component parts including but not limited to:    1. A backlighting unit (in the case of a transmissive display) to supply even, wide angle illumination to the panel.    2. Control electronics to receive digital image data and output analogue signal voltages for each pixel, as well as timing pulses and a common voltage for the counter electrode of all pixels. A schematic of the standard layout of LCD control electronics is shown in FIG. 1 (see, E. Lueder, Liquid Crystal Displays, Wiley and Sons Ltd., 2001).    3. A liquid crystal (referred to herein as LC) panel, for displaying an image by spatial light modulation, includes two opposing glass substrates, onto one of which is disposed an array of picture element electrodes and an active matrix array to direct the electronic signals, received from the control electronics, to the picture element electrodes. Onto the other substrate is usually disposed a uniform common electrode and colour filter array film. Between the glass substrates is contained a liquid crystal layer of given thickness, usually 2-6 μm, which may be aligned by the presence of an alignment layer on the inner surfaces of the glass substrates. The glass substrates will generally be placed between crossed polarising films and other optical compensation films to cause the electrically induced alignment changes within each picture element region of the LC layer to produce the desired optical modulation of light from the backlight unit and ambient surroundings, and thereby generate the image.
The aforementioned picture elements are commonly referred to as pixels, where each pixel usually consists of a plurality of sub-pixels. Typical LCDs have an RGB stripe geometry, where the pixels are square in shape with three sub-pixels, one red, one green and one blue all of which are shaped as vertical stripes. However, multi-primary displays with pixels containing four or more sub-pixels, for example one red, one green, one blue and one white, are becoming more common.
Multi-primary displays have been produced with the aim to expand the range of displayable colours (Proceedings of the IDW'09, 2009, pp 1199-1202). Multi-primary displays with one red, one green, one blue and one white sub-pixel have been developed with the aim of improving the display brightness and therefore efficiency (SID'08 Digest, pp 1112-1115). Multi-primary displays have also been produced with the aim of simultaneously increasing brightness and increasing the ability to render fine image features on a sub-pixel level (IMID '05 Digest, pp 867-872). Multi-primary displays with one red, one green, one blue and one yellow sub-pixel have also been developed; these displays show enhanced brightness, increased colour gamut, and increased sub-pixel rendering ability (SID'10 Digest, pp 281-282).
As multi-primary displays have more than three types of colour sub-pixels, for many chrominance and luminance values, there may be multiple configurations of individual data values supplied to the colour sub-pixels which produce the same luminance and chrominance overall. The different sets of data values that produce the same overall luminance and chrominance are known as metamers. A method for selecting the most desirable metamer, based on sub-pixel rendering considerations, is described in US 2010 0277498 (published 4 Nov. 2010).
There have been many other advances in LCD technology resulting in very high performance displays with improved metrics such as display area, brightness, image contrast, resolution, bit-depth and response time. However viewing angle characteristics remain poor for many types of LCDs. To achieve good viewing angle characteristics the relationship between the input image data value for a given pixel and the observed pixel luminance, often called the gamma curve, must change as little as possible with viewing angle. The gamma curve of the display is determined by the combined effect of the data-value to signal voltage mapping of the display driver, and the signal voltage to luminance response of the LC panel.
One problematic viewing characteristic is contrast inversion. Contrast inversion occurs when a pixel which has been switched to have a higher luminance than another pixel when observed from a direction normal to the surface of the display (referred to herein as on-axis) does not remain a higher luminance at all viewing angles and consequently the displayed image can appear to invert with changing viewing angle. Several technologies have been developed to solve the contrast inversion problem. For example, displays have been produced with angular compensation films such as the splayed-discotic Wide-View film for Twisted Nematic (referred to herein as TN) displays, multidomained pixels for Vertically Aligned Nematic (referred to herein as VAN) displays, In-Plane Switching (referred to herein as IPS) mode displays and improved electrode geometries.
A second problematic viewing characteristic is the change in perceived colour with viewing angle; this is commonly known as colour shift. Colour shift results from the fact that the amount of luminance variation of a pixel with viewing angle is a function of the on-axis luminance of the pixel. Consequently, in an RGB stripes display where the three sub-pixels have different luminance values, the relative difference in luminance between the three colour components can change with viewing angle. Whilst the contrast inversion problem has widely been solved, colour shift remain a problem for many types of LCDs.
For reasons of clarity, the following examples used to illustrate the colour shift effect and the descriptions of the embodiments to reduce the effect will be directed toward VAN mode LCD displays, with 8 bit per colour gradation control. The problem of colour shift with angle is not restricted to VAN mode displays or displays of any particular colour depth, nor is the applicability of the embodiments described herein, so this should not detract from the scope of the invention, which is applicable to any LCD which exhibits colour shift with angle.
FIG. 2 shows the measured angular dependence of the luminance of a multidomained VAN mode LCD in a mobile phone display, at shades of grey from input data level=0 (black) to 255 (white) in steps of 32. FIG. 3(a) shows the points of FIG. 2 at 0° and 50° inclination to the right hand side (horizontal in the orientation in which the display is normally observed) plotted against the input data level. The on-axis curve is the display gamma curve which is designed to approximately follow the relationship
      L          L      max        =            (              D                  D          max                    )        γ  where L is the output luminance, for a given data level D, and γ (gamma) is the power relating the two when each is normalised to their maximum value. The gamma value is typically engineered to be in the region of 2.0 to 2.4, and is approximately 2.3 for the display shown in FIGS. 2 and 3.
FIG. 3(b) shows the luminance of the display at 0° and at 50° as a function of the on-axis luminance, both are normalised to their maximum values.
From the figures it can clearly be seen that the typical behaviour for a VAN mode display is for mid-grey levels to appear disproportionately bright when viewed off-axis. This is further illustrated in FIG. 4, which shows the luminance as a function of viewing angle, normalised to the luminance of the data=255 state at each angle, for the same VAN mode display displaying input data values equal to 255, 160 and zero. From this figure, it can be seen that if a pixel was input with data=255 to the red colour sub-pixel, with data=160 to the green colour sub-pixel and with data=0 to the blue colour sub-pixel, on-axis, the ratio of normalised luminances is approximately 1:0.35:0 for R:G:B, which would result in an orange coloured appearance for the pixel. However, when viewed from a 50° inclination, the ratio of colour components is approximately 1:0.77:0.03, which would result in a yellow appearance for the pixel. This is the cause of the colour-shift with viewing angle, and it can be seen that, for VAN mode displays in particular, the degree of colour shift is greatest for colours which are composed of one colour component near maximum luminance, and one or two colour components in the mid-luminance range.
Several technologies have been developed to mitigate the effect of colour shift. The most effective of these utilise a split sub-pixel architecture, whereby each colour sub-pixel in the display consists of two or more regions. Each sub-pixel region has a different luminance, one higher than the other; consequently each sub-pixel region has a different variation in luminance with viewing angle. Sub-pixel region luminance values are chosen so that the average on-axis luminance of the sub-pixel regions has the desired overall luminance and so that the average change in luminance with viewing angle of sub-pixel regions is less pronounced than each region taken individually.
This method is known as partial spatial dither or digital halftoning, and can be implemented using a capacitive potential divider between the regions of the split sub-pixel, as described in U.S. Pat. No. 4,840,460 (published 20 Jun. 1989), and U.S. Pat. No. 7,474,292 (published 6 Oct. 2005), or it can be implemented by using an additional source line per colour sub-pixel, such that each of the two regions of the sub pixel receives an independently controlled signal voltage when they are activated by a common gate line. This second implementation is described in U.S. Pat. No. 6,067,063 (published 23 May 2000). The two general approaches are also summarised in U.S. Pat. No. 7,079,214 (published 18 Jul. 2006), in addition this patent also describes how to optimise the relationship between the voltages applied to the brighter and darker sub-pixel regions so as to achieve reduced colour shift.
However, there are negative aspects to the hardware split sub-pixel architecture. Added pixel electronics are required which increases the cost of the display and the method is not applicable to high resolution, small area displays.
It is not necessary to have a split sub-pixel architecture to implement such a method. The technique can effectively be implemented in software, or in the LCD control electronics, and applied to any existing colour display by adjusting the luminance of whole colour sub-pixels up and down alternately, either in the spatial or temporal domain, to create the same effect at the expense of the effective resolution of the display. Luminance is effectively transferred between identical sub-pixels of neighbouring pixels; this is done in such a way so as to ensure the average on-axis luminance of the neighbouring pixels is unchanged whilst the average colour shift is improved. This is described in U.S. Pat. No. 6,801,220 (published 17 Oct. 2002), U.S. Pat. No. 7,113,159 (published 7 Aug. 2003), U.S. Pat. No. 5,847,688 (published 8 Dec. 1998), U.S. Pat. No. 7,250,957 (published 31 Jul. 2007), US 2004 0061711 (published 1 Apr. 2004), US 2010 0156774 (published 24 Jun. 2010) and U.S. Pat. No. 7,764,294 (published 10 Aug. 2006).
In U.S. Pat. No. 6,801,220, this is implemented on an RGB display by an image processing method in which the image data input to the LCD is manipulated by means of a look-up table (referred to herein after as LUT), so that for each input data level, a pair of output data levels is provided which, when displayed by neighbouring pixels on the LCD, are averaged by the eye of the viewer, assuming sufficient display resolution and viewing distance, to appear the same as if the original input data level were displayed on both pixels. The image processing method therefore alternates spatially across the display which of the pair of output data values is applied to each pixel for a given input data value.
U.S. Pat. No. 7,113,159 describes a liquid crystal display device composed of three sub-pixels, red, green and blue, with an excellent graduation curve with wide viewing angle. The wide viewing angle is achieved by adjusting the luminance of the sub-pixels up and down in the temporal domain. In other words, the frames in one pixel display respectively different graduations. The frame switching performed at a sufficiently high speed causes a colour mixture to occur by image persistence, and the colour appears a middle luminance to the eye. The patent also describes a type of hardware split sub-pixel architecture but highlights two problems with the method, the first being the increase in pixel electronics and the second being the reduced transmittance of the sub-pixels. A non-hardware split solution to these problems is suggested whereby a white sub-pixel is added to each pixel. The viewing angle is then improved by correcting the graduation characteristic with respect to the combination of red, green and blue.
However, there are also negative aspects to the software split sub-pixel architecture. Whilst no added pixel electronics are required as in the hardware split sub-pixel architecture and the software method can be applied to high resolution, small area displays the resulting images do suffer from an effective loss in luminance resolution. The chrominance of each individual pixel may also differ from its original value which can lead to colour artefacts in the resulting image. U.S. Pat. No. 6,801,220 states that the halftoning pattern used will have the same overall appearance as the original image only if the image content changes gradually from pixel to pixel. If the image content changes sharply from pixel to pixel, then the halftoning pattern is disrupted. For example, the patent states that a 2×2 sub-pixel pattern can be used where the periodicity of the pattern is two pixels in both the horizontal and vertical directions. The brightened or darkened regions consist of either a single sub-pixel or a pair of sub-pixels. FIG. 5 shows the green/magenta colour arrangement for this pattern. If this pattern is applied to a one pixel by one pixel chequer board image the halftoning pattern is disrupted and colour artefacts are visible. FIGS. 6(a) and 6(b) illustrate how an original image with a 1×1, grey/black chequer board appears when the green/magenta colour arrangements of the aforementioned pattern is applied to the image. Green and magenta artefacts are visible. Single pixel diagonal lines also suffer from similar colour artefact problems. (The “+” and “−” signs in the sub-pixels of FIGS. 5, 6(a) and 6(b) indicate the polarity of the voltage applied to that sub-pixel in one frame, with the polarity of the voltage applied to a sub-pixel being reversed from one frame to another as is common for driving an LC display.)
US 2010 0156774 describes a frame inversion drive method with no apparent resolution loss in either luminance or chrominance for static images. In this drive method the bright-dark spatial chequer pattern is imposed in the image within each frame, but the chequer pattern is inverted with each frame change. To the observer, the image of each frame appears identical due to the spatial averaging of the eye making it impossible to discern which of a pair of pixels has been made brighter or darker within a given frame. The key advantage of this frame inversion drive method is that although the macroscopic appearance of each frame, for a static input image, is identical, each pixel is made to change in brightness from frame to frame so as to provide an average luminance over time equal to the desired luminance corresponding to the input data value to that pixel. Therefore, although within each frame a resolution loss is incurred due to the data modifications applied imposing the bright-dark chequer pattern, over a period of two frames or more, each individual pixel provides the correct average luminance, so no apparent resolution loss is incurred.
Whilst it is true that the aforementioned frame inversion drive method can recover some of the luminance resolution loss and no colour artefacts are visible in static image, movement of the eye around the display or blinking can lead to an instantaneous glimpse of the display and consequently at this moment the loss in resolution is still visible; although it must be noted that colour artefacts are not visible during an instantaneous glimpse of the display.
Despite the fact that colour artefacts are not visible when the frame inversion drive method is applied to static images, colour artefacts are visible in some moving images. For example when the modification pattern illustrated in FIG. 5 is applied to a one pixel by one pixel chequer board moving horizontally at a rate of one pixel per frame, colour artefacts are visible even when the frame inversion drive method is applied.
US 2010 0156774 describe a method that can be used to solve the problem of coloured artefacts in moving images. The method involves preventing any modifications being performed on the input image in regions where colour artefacts would result. Whilst the method does prevent coloured artefacts in both static and moving images, the method has the disadvantage that any pixels where the modifications have been prevented do not have any improvement in their off-axis appearance. As a result, identical input pixels where modifications have been applied to one and not to the other appear different to an off-axis viewer. The method also has the additional disadvantage that extra resource is required to implement it. Consequently it would be preferable that no colour artefacts occur in the first place.
It is therefore clear that a requirement exists for an optimised method of reducing colour shift with viewing angle in LCDs where there is no luminance resolution loss or reduced luminance resolution loss compared to the existing methods as well as no colour artefacts for both moving and static images.